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Date:      Wed, 18 Jan 2012 10:09:23 -0500
From:      John Baldwin <jhb@freebsd.org>
To:        davidxu@freebsd.org
Cc:        svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org
Subject:   Re: svn commit: r230201 - head/lib/libc/gen
Message-ID:  <201201181009.23221.jhb@freebsd.org>
In-Reply-To: <4F1629D5.4020605@gmail.com>
References:  <201201160615.q0G6FE9r019542@svn.freebsd.org> <201201170957.47718.jhb@freebsd.org> <4F1629D5.4020605@gmail.com>

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On Tuesday, January 17, 2012 9:09:25 pm David Xu wrote:
> On 2012/1/17 22:57, John Baldwin wrote:
> > On Monday, January 16, 2012 1:15:14 am David Xu wrote:
> >> Author: davidxu
> >> Date: Mon Jan 16 06:15:14 2012
> >> New Revision: 230201
> >> URL: http://svn.freebsd.org/changeset/base/230201
> >>
> >> Log:
> >>    Insert read memory barriers.
> > I think using atomic_load_acq() on sem->nwaiters would be clearer as it would
> > indicate which variable you need to ensure is read after other operations.  In
> > general I think raw rmb/wmb usage should be avoided when possible as it is
> > does not describe the programmer's intent as well.
> >
> Yes, I had considered that I may use atomic_load_acq(), but at that time,
> I thought it emits a bus locking, right ? so I just picked up rmb() which
> only affects current cpu. maybe atomic_load_acq() does same thing with 
> rmb() ?
> it is still unclear to me.

atomic_load_acq() is the same as rmb().  Right now it uses a locked
instruction on amd64, but it could easily switch to lfence/sfence instead.  I
had patches to do that but I think bde@ had done some benchmarks that showed
that change made no difference.

-- 
John Baldwin



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